PIP: In underdeveloped countries, public health departments have initiated programs for the reduction of child and maternal morbidity. In order to uncover the critical points of the action and to evaluate the impact of these programs, information on the demographic characteristics of the population under study and on the way to collect data on programs accomplishments is necessary. 3 major elements to be surveyed include effects, events and time. The Lexis diagram is used to represent events in time. Different types of surveys include 1) unique punctual survey with retrospective questions; 2) several punctual surveys without retrospective questions; 3) unique punctual surveys with retrospective questions; and 4) several punctual surveys with retrospective questions. For a prolonged registration of the effects, a personal record should be established, containing a history following the initial event. The combined approaches are a melting pot of all the different ways of collecting data. Applications related to health include evaluating demographic characteristics (punctual observation on hygiene and nutrition); child development (prospective study); and health of the pregnant woman (continual observation). Monitoring of the state of health of a population needs an infracture of observation relatively developed. An integrated system of collecting is more convenient for obtaining detailed data concerning variables of changes difficult to observe by classical means.